Breast pumps are configured to pump breast milk into a receptacle. Conventional breast pumps attach to a plastic feeding bottle. The pumped milk is deposited directly into the feeding bottle. Certain breast pump systems are configured to pump directly into a flexible storage bag. Many of these systems require the bag to be contained in a rigid vessel during pumping. In some of the systems, the bag hangs directly from the pump or an adaptor connected thereto. One adaptor for securing a bag to a breast pump includes a cylindrical conduit with diametrically opposed mounting hooks extending radially outward from the conduit. Each of the mounting hooks includes a radially outwardly projecting portion and an upwardly extending portion. To secure a bag to the adaptor, holes in the bag are threaded over the upwardly extending portion of the mounting hooks and onto the radially outwardly projecting portion of the mounting hooks. The bag hangs down from the radially outwardly projecting portion of the mounting hooks during use. Since the mounting hooks in this design project radially outwardly from the conduit, the bag opening must be larger than the conduit so that the holes in the bag can be threaded onto the hooks. As a result, the bag does not closely conform to the conduit, which can permit air and contaminants to enter the bag. In addition, because the mounting hooks extend in two directions (i.e., radially outward and upward) removing the bag from the adaptor when it is filled with liquid can be difficult. Moreover, the positioning of the adaptor with the bag makes it impossible to seal the bag prior to removal of the bag from the adaptor.